One dose of the Pfizer vaccine could be enough to protect millions of people who have already had Covid, research by Public Health England suggests.
Scientists said the findings could "potentially accelerate vaccine rollout" if those going for jabs were offered tests to see whether they had antibodies first.
One of the studies, led by researchers at University College London and Public Health England (PHE), tracked 51 health workers in London who had undergone regular tests for antibodies and infection since March.
Roughly half had been infected, and a single dose of the Pfizer vaccine increased their antibody levels more than 140-fold from their peak levels before being inoculated, the study said. This appeared to give them more robust protection than two doses of the vaccine did in people who had never been infected.
"This could potentially accelerate vaccine rollout," the scientists wrote, suggesting that speeding up the programme could also slow the spread of new virus variants. The paper said "wider coverage without compromising vaccine-induced immunity could help reduce variant emergence".
There have been more than four million confirmed cases of Covid in the UK, though this is likely to far underestimate the number of people who have been infected.
The second study, led by scientists at Imperial College London, measured the immune responses of 72 health workers who were vaccinated in late December.
One third showed signs of having previously been infected. For them, one dose of the Pfizer vaccine stimulated "very strong" antibody responses, the study said, as well as "very strong T-cell responses", referring to another arm of the immune system.
Both studies involved only the Pfizer vaccine. Scientists in the UCL study suggested that any change in the vaccination strategy would require those who previously tested positive for Covid to be given blood tests in the weeks before they became eligible for a jab to ensure that they had sufficient antibodies.
Such moves at this stage in the vaccination programme would be likely to add to its complexity, making such a strategy unlikely.
Healthcare workers are the most likely to have already had Covid, but the vaccine programme has now offered jobs to all in this group, meaning it is too late to offer tests before the first jab, which would itself produce antibodies.
Scientists also stressed that it is not yet known how long the post-vaccine immune response among previously infected people would last after being given one jab.
A Department of Health and Social Care spokesman said: "With over 19.1 million people now receiving their first dose, the vaccination rollout continues to rapidly expand, helping to save lives and reduce hospitalisations. Data shows both vaccines currently being used are highly effective after two doses, and our programme remains unchanged."
It came as Prof Jonathan Van Tam, the deputy chief medical officer, said he was concerned that the public might be tempted to bend the Covid rules, telling a Downing Street press conference: "I do worry that people think it's all over."
Speaking after Matt Hancock, the Health Secretary, warned that the decline in cases is slowing, Prof Van Tam said: "Please don't be tempted to think, 'well, one home visit might be alright now the weather is getting better, it's going to be a nice weekend – one small gathering in your house won't really matter'. I'm afraid it does.
"This is all going very well but there are some worrying signs that people are relaxing, taking their foot off the brake at exactly the wrong time. Do not wreck this now. It is too early to relax. Just continue to maintain discipline and hang on just a few more months."